So I want to make a summer shandy, can I just put fruit in the second fermentation stage instead of juice?

One difficulty you'll have with getting that signature shandy flavor when adding fruit to secondary is that the sweetness (sugar) of the juice will get converted to alcohol by the yeast, leaving you with mostly aroma, and a little flavor. A lot of people don't recognize how much sugar plays into the overall taste of the fruit. Without the sugars, it is not nearly as appealing as we may think. In my experience, the aroma tends to drop off over time as well. This is why people tend to add a lot of fruit, especially of the citrus variety.

The important thing to keep in mind with this is to really recognize how vital aroma is to our perceived tastes. I have a berlinner weisse on tap right now (mild on the lactic tartness) that I added 3 pureed pineapples to in secondary. The sugars from the pineapple fermented out, leaving you with only the faintest hint of actual pineapple flavor. Within the first 2 weeks, the aroma was nothing but pineapple. Friends (untrained palettes) would try the beer and remark that it tastes like pineapple juice, even though the actual pineapple flavor was very, very mild (likely they perceived the lactic acid tartness as a character of the pineapple additions and not the sour mash, I didn't want to downplay my beer or confuse them though). It's been in the keg for close to a month now, and I'm beginning to see a drop off in aroma. Without having that pineapple punch in the sniffer, it becomes more obvious just how slight the pineapple flavor was, when before, the very, very strong pineapple aroma would trick people into thinking it has more pineapple flavor than it actually did.

That's why people blend in things like lemonade and other juices when they pour it as brewchez recommended, as to avoid fermenting out the sugars. Without verifying, I suspect a lot of people who blend their shandies for production either filter out the yeast, or kill the yeast before blending so that the sugars can't ferment out.

You can certainly try it. That's the major advantage of homebrewing.

However, just because these beers are coming prepackaged nowadays doesn't mean that's the way its done in the place of origin.

These things evolved really as beer cocktails.

I think its far better to just add the lemonade to the beer in the glass. That way you have great beer to begin with and don't have to drink it all as shandy. You can also dial in the amount of dilution to fit your own taste.

(Or try a Radler which is typically lemon/lime/citrus soda and lager beer together).